1. Field of the Invention
The present invention pertains to optical wavelength division multiplex transmission systems in general. Specifically, the present invention pertains to adjustment of transmitted signal power levels in optical wavelength division multiplex transmission systems.
2. Discussion of the Related Art
Owing to the wavelength dependency of optical amplifiers, optical wavelength-division multiplex transmission systems have losses in the transmission fibers and in passive optical components as well as attenuation levels which differ due to non-linear effects such as signal coupling resulting from stimulated Raman scatter in general for the various signals and channels. In an optical transmission path which comprises a number of path sections having a number of fiber amplifiers, these effects can become additive. As a consequence of this, at the receiving end, the weaker optical signals are no longer detected without faults by the optical receiver since their levels are too low or since their optical signal-to-noise ratio (OSNR) is too low. On the other hand, the maximum permissible input level of the optical receiver may be exceeded by a signal which is attenuated to a lesser extent.
One method, which is used in existing optical transmission systems, for compensating for the different levels or OSNR values is corresponding initial compensation at the transmission end, which is referred to as preemphasis. In this case the level or OSNR distribution of the channels/signals at the reception end is measured using an optical spectrum analyzer, and the level at the transmission end is raised for signals which arrive with severe attenuation at the receiver, while the level of more powerful signals is correspondingly reduced, to ensure that all the received signals have the same power level, also referred to as level balance, or the same optical signal-to-noise ratio (OSNR balance) at the reception end. The raising or lowering of the transmitted signal level for each channel or for each transmitted signal is generally selected such that the total transmitted signal power level, that is the total of the power levels of all the transmitted signals and of the total signal, remains unchanged at the start of the optical path or does not exceed a maximum value.
Suitable algorithms for level and OSNR balancing are described in the article “Equalization in Amplified WDM Lightwave Transmission Systems” appearing in IEEE Photonics, Technology Letters, Vol. 4, No. 8, August 1992, pages 920 to 922.
However, disadvantages can occur if exactly implemented level or OSNR compensation is used. One such disadvantages is that, owing to the wavelength dependency of the path loss, complete level balancing for the reception end can lead to an excessively high level dynamic range at the transmitting end. Hence, an excessively large quotient between the maximum and minimum channel power level can occur. There is then a risk of signals with a raised transmitted power level being distorted by non-linear effects in the fibers and/or of transmitted signals with a greatly reduced level actually falling below the minimum input power level of an optical amplifier, which would result in considerable signal distortion due to noise.
Complete OSNR balancing for the reception end can also lead to an excessive level dynamic range at the transmitting end. In addition, there is a risk of the maximum permissible input level range of one or more of the connected optical receivers being exceeded or undershot.
A method and arrangement for adjusting identical signal levels is shown in U.S. Pat. No. 5,815,299. In the method, the average level of all the transmitted signals and the level of the weakest signal are determined. The other signals are attenuated as a function of the difference between the average level and the level of the weakest signal. However, this reduces the average level and thus the quality of all the other channels. Such a method admittedly leads to identical levels in all the channels, but does not optimally utilize the maximum possible dynamic range and thus does not achieve optimum transmission quality or optimum range.
In general, exact level balancing at the reception end is not required, since the connected optical receivers have a considerable level dynamic range in which they operate optimally. In the same way, exact OSNR balancing is not required, provided appropriate system margins are available. In this case, a method which considers only the dynamic range of the transmitted signals would be optimal. Since, systems of this type operate at an optimum or maximum permissible total power level, it is advantageous for this level to remain constant in any compression of individual transmitted signal power levels which may be required.
For OSNR balancing however, the maximum permissible dynamic range at the reception end must also be checked, and if necessary, the received signal power levels are adapted by compression. This is done by changing the power level of the individual transmitted signals, and compliance with the transmission-end dynamic to range must be checked once again and, if necessary, changed.